As a result of an increasingly further-reaching quality control of the raw materials for the polyamide fibers and of a very well-developed technology for spinning, polyamides, in particular nylon-6, are now being successfully spun for textile yarns at spinning speeds of 6000 metres per minute and sometimes even higher.
At such extreme speeds, the spinning process is very susceptible to variations, especially to variations in the melt viscosity of the polyamide presented and even small variations result in fiber rupture. In the case of failure of one spinnerette, an appreciable difference in molecular weight and, therefore in the properties of the fiber from the remaining spinnerets of a production line will occur.
However, the limit of what is possible appears to have been reached with the present generation of polyamides, in particular nylon-6, and a breakthrough is necessary to achieve a further increase in the spinning speed or a further reduction in faults.
The inventors have achieved such a breakthrough by spinning the fibers of branched polyamides completely contrary to the prevailing teaching on polymers for fiber spinning. Said prevailing teaching is evident, inter alia, from the following literature citations.
In their authoritative series of publications entitled "Studies of polymerisation and ring formation" in Part XV, Artificial fibers from linear condensation superpolymers, J. Am. Chem. Soc., 54, 1579-87 (1932), Carothers and Hill make an attempt to provide a basis and, in doing so, refer to the importance of the molecular length for the strength. Furthermore, it is stated that crystallization is important and that "Linear condensation polymers are quite generally crystalline unless bulky substituents are present to destroy the linear symmetry of the chains".
Later publications and textbooks also emphasize the importance of linear molecules and consider the presence of side chains as troublesome, if not impossible. Examples of this are found in R. W. Moncrieff, Artificial Fibers, National Trade Press, London (1950), pages 30 and 48 and, in particular, on page 194 "It should be noted that the molecules of nylon are long and straight, that there are no side-chains or cross linkages." J. G. Cook in Handbook of Textile Fibers, II Man-made Fibers, 5th ed. Mirrow, Shildon, UK (1984), pages IX and X states "The molecules of a fiber are thus in shape very similar to the fiber itself. And just as the fiber bestows its characteristics on the yarn of which it forms a single strand, so does the fiber derive its properties from the thread-like molecules of the substance it is made. . . . But it is only the very special types of long molecule that are able to form fibers. They must, for example, be fairly regular in shape with a repeating pattern of atoms in the molecule. They must not have large pendant groups of atoms sticking out from the sides."
In summary, it follows from the above that the existing teaching is that, in the synthesis of polyamides for use as fiber material, the occurrence of branching must be avoided.
The inventors have, however, found very surprisingly that a fiber having comparable mechanical properties to those of a fiber composed of linear polyamide is obtained with heavily branched polyamide in a very reproducible way with a lower sensitivity to faults, as for instance fiber rupture during spinning.
A further advantage of the fiber made of branched polyamide according to the invention is that it can be obtained with a lower melt temperature at the same spinning speed or a higher spinning speed at the same temperature as the fiber composed of linear polyamide having the same relative viscosity.